Stallord

In the Arbiter's Grounds, you'll find this fossilized creature-thing called Stallord.

Stallord will appear once you near the skull in the center.

Use the spinner on the railing and ride around to the boss's backside (make sure to jump off if am enemy spinner nears on the railing). Head towards the boss's backbone and attack by Spinning when near to destroy the vertebrates one by one. Just watch out for the skeletons that pop out of the ground, as they'll act like pinball bumpers preventing you from getting to the boss. After three vertebrates are destroyed, the boss be reduced to a mere skull.

Raise a pillar in the center with the spinner. After which, the skull will come to life and knock you off the platform. There are two tracks: one around the center column and the other on the outside of the arena. You can hop between the two by pressing B. Hop onto the inside track and ride it back up to the boss. When you see him he'll begin to launch fireballs. Avoid them by jumping between the two tracks. Once he gets close, jump again to knock him to the ground and slash him with your sword. Spin attacks and jump strikes would work best (do the spin attack, and while it's recharging, keep doing jump strikes). After several attacks, he'll be defeated.